ABSTRACT Mounting evidence suggests that autoantibodies can antecede overt clinical disease, with anti-SSA/Ro among the earliest reactivities. Yet, predicting who remains protected from overt disease or who will progress would be a game-changing discovery for understanding the pathogenesis of Systemic Lupus Erythematosus (SLE). Mothers in the Research Registry for Neonatal Lupus (RRNL) represent a unique population at risk for overt clinical autoimmunity. Despite the presence of high titer anti-Ro antibodies potentially pathogenic to the developing fetus, many women are asymptomatic, with autoimmunity identified solely based on disease in their offspring. Currently, of 521 RRNL mothers, 283 were asymptomatic, or had only minimal symptoms, at the time of the birth of the NL child, whereas a subset developed SLE. The intertwining of genetic variation in the HLA locus with the capacity to undergo shifts toward a pathologic gut microbiome may, in part, explain progression from benign to pathologic autoimmunity and overt SLE. Alternatively, other HLA variants may instead associate with a protective microbiome that halts progression so that a state of benign autoimmunity persists. In Specific Aim 1, associations will be sought between host genetics and anti-Ro NL disease status. We will genotype an estimated 297 RRNL mothers using an HLA next generational sequencing approach that makes no a priori sequence assumptions. The HLA alleles will be informative to distinguish asymptomatic anti-Ro-mothers (particularly those remaining so for >6 years) from those who initially or quickly progressed to classified SLE. In Specific Aim 2, associations will be sought between gut microbiome taxa in the context of HLA and anti-Ro NL maternal disease status, as we address the hypothesis that HLA is a central driver of pathobiont associations that contribute to the pathogenesis of overt SLE. A planned longitudinal analysis of the asymptomatic RRNL mothers will be highly informative and expected to capture women who transition to established SLE. In Specific Aim 3, we will elucidate the relationships of serological and cellular immunity (and CD4 T-cell subsets) and anti-Ro NL maternal disease status. Newly generated data will be continuously shared and discussed with Drs. Silverman (Project 2) and Reizis (Project 3) to further explore associations in the context of circulating and fecal IgA and anti-DNASE1L3 sensitive reactivity with microparticles. A molecular basis for the specific microbial taxa that are coated with sIgA will be evaluated in context of a mimicry hypothesis that considers cross-reactivity between T cell epitopes and bacterial peptides. From a public health perspective, it is anticipated that conserved patterns or blueprints will be identified for host immune systems that are best suited to arrest autoimmunity at a benign preclinical state or conversely that favor progression to overt SLE and tissue injury. Identification of one or more predictive biomarkers that associate with a persistently asymptomatic state could also serve as the ?target to achieve? while a permissive factor might represent a ?target to inhibit? in an established SLE patient.